A copper foil is generally used as a current collector of a secondary battery. As the copper foil, a rolled copper foil obtained by rolling processing is mainly used, but fabricating cost of the rolled copper is high and it is difficult to fabricate a copper foil having a wide width. Further, since the rolled copper foil uses lubricating oil during the rolling processing, adhesion with an active material may be degraded by a contamination of the lubricating oil, so that a charging and discharging cycle characteristic of the battery may be degraded.
A lithium battery accompanies a change in volume during charging and discharging and a heating phenomenon according to overcharging. Further, in order to improve adhesion with an electrode active material and, in relation to expansion and contraction of an active material layer according to the charging and discharging cycle, in order to prevent wrinkle, fracture, and the like from being generated in the copper foil, which serves as a current collector, with little influence from a copper foil base material, surface roughness of the copper foil needs to be low. Accordingly, a highly elongated, highly strong, and low rough copper foil, which is capable of resisting to a change in volume and a heating phenomenon of the lithium battery and has excellent adhesion with an active material, is required.
Further, according to a demand for a light, thin, short, and small electronic device, in order to increase a degree of integration of a circuit within a small area according to high performance, compactness, and lightness, a demand for fine wiring of a semiconductor mounted substrate or a main board substrate is increased. When a thick copper foil is used for fabricating a printed wiring board having a fine pattern, an etching time for forming a wiring circuit is increased and lateral wall verticality of a wiring pattern is degraded. Particularly, when a line width of the wiring pattern formed by etching is small, the wiring may be disconnected. Accordingly, in order to obtain a fine pitch circuit, a thinner copper foil is demanded. However, a thickness of a thin copper foil is limited, so that mechanical strength of the thin copper foil is weak, so that a frequency of the generation of a failure, such as wrinkle and bending, is increased during fabricating the wiring substrate.
Further, a plurality of terminals of an Integrated Circuit (IC) chip is directly bonded to an inner lead disposed in a device hall positioned at a center portion of a product in a semiconductor packaging substrate for Tape Automated Bonding (TAB) used in a Tape Carrier Package (TCP) and the like, and in this case, a current is made to momentarily flow in the inner lead by using a bonding device to heat the inner lead and apply a predetermined pressure to the inner lead. Accordingly, the inner lead formed by etching the electrodeposited copper foil is elongated and stretched by a bonding pressure.
Accordingly, a copper foil having a small thickness, high mechanical strength, a high elongation property, and low roughness is required.